1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, or a facsimile machine, for forming an image onto a sheet and, more particularly, to a sheet feeding device provided in the image forming apparatus.
2. Description of the Related Art
Hitherto, for example, copying machines, printers, and facsimile machines have been used as image forming apparatuses.
In general, copying machines are machines which read image information from an original and form an image on a sheet in accordance with the read image information. In recent years, copying machines capable of being used in communications have been made available. Such copying machines can receive read image information transmitted to the copying machines from an external device.
In general, printers are machines which form an image on a sheet based on image information transmitted from an external device, such as a computer. Facsimiles are, in general, machines which read an original, and which can be used in communications to transmit read image information to an external device and to form an image on a sheet in accordance with information received from an external device.
Image forming apparatuses of the aforementioned types are provided with a sheet feeding device for proper sheet feeding.
A sheet feed cassette of a sheet feeding device generally includes an inside plate. In the sheet feed cassette, a plurality of sheets are loaded in a stack on the inside plate, and a side regulating member and a rear end regulating member are moved and positioned in accordance with the size of the sheets. The inside plate is pushed upward by means of a biasing spring to bring the topmost sheet of the sheet stack into contact with a sheet feed roller, which rotates to feed the sheets one sheet at a time to an image forming unit to form an image on each of the sheets by a predetermined image forming means.
The rotational center of the inside plate is located at about the center of the sheet feed cassette. As the number of sheets in the cassette decreases, the inside plate is biased by the biasing spring, causing one end of the inside plate and one of the ends of each of the loaded sheets to move upwards while they move through a circular arc.
In addition, a separating means is provided for separating the sheets so that they are fed from the sheet feed cassette one sheet at a time. Examples of separating means used in, for example, a low-speed sheet feeding device which feeds 15 sheets per minute, include a pawl separating means and a friction separating means using a separating pad.
The pawls of the pawl separating means are disposed on both sides of one end of the cassette in such a manner as to allow only the topmost sheet of the stack of sheets to be transported by the sheet feed roller. Since the pawls are provided on both sides of one end of the cassette, the cassette cannot be easily replenished with sheets, and sheets of various sizes cannot be used. Therefore, the pawls must be formed, for example, into shapes that overcome the aforementioned problems, so that there is not much latitude in setting the optimum conditions, imposing limitations upon the designing of the pawl separating means.
The friction separating means, which allows the use of sheets of various sizes with relative ease, is widely used.
However, the above-described conventional sheet feeding device has the following problems.
The friction separating means comprises a separating pad provided at one face of the cassette, and a sheet feed roller provided at the upper portion of one end of the cassette. The sheets are separated by means of the separating pad and the sheet feed roller in order to feed them one sheet at a time. However, depending on the amount of shifting of the end of a sheet, the friction separating means having the above-described construction may fail to feed the sheets properly.
In particular, as the number of loaded sheets decreases, the ends of sheets tend to get shifted, so that the sheet feeding device frequently fails to feed the sheets properly.
A description will now be given of what tends to cause one end of a sheet to shift as the number of the loaded sheets decreases.
The inside plate of the cassette is rotatably supported near the center of the cassette such that one end of the inside plate can move through a circular arc.
One end of the sheets on the inside plate similarly move through a circular arc. When there are a large number of sheets, the sheets are set substantially horizontally. As the number of sheets decreases, the inside plate and the sheets move rotationally such that their ends move upward.
When the length of the sheet in the sheet transporting direction is defined as L, and the inside plate is tilted by an angle of .theta. degrees, the horizontal distance is equal to L x cos .theta.. The larger .theta. becomes, the smaller the horizontal distance from the rotational center.
In other words, the smaller the number of sheets, the larger the distance between one end of the sheets and the sheet feed roller, so that the sheets tend to be improperly fed.
When the sheet feed roller is formed with a small diameter in order to produce a small sheet feeding device, the ends of the sheets tend to get separated from the sheet feed roller by a much greater distance, resulting in more frequent shifting of the ends of the sheets, so that the sheets tend to be improperly fed.
A description will now be given of the problems that occur when the horizontal distance from the rotational center decreases as .theta. increases.
The cassette is ordinarily provided with a side regulating plate and a rear end regulating plate for properly setting a sheet in position in accordance with the size of the sheet. Allowing for, for example, variations in sheet dimensional tolerances, or expansion of sheets caused by temperature changes and moisture absorption, the side regulating plate and the rear end regulating plate are set 1 to 2 mm beyond the edges of the sheets.
A universal cassette is available, in which the positions of the side regulating plate and the rear end regulating plate can be changed freely in accordance with sheet size. In a universal cassette, however, it is quite difficult to set the side regulating plate and the rear end regulating plate in exact correspondence with the sheet size, so that the user may set the side regulating plate and the rear end regulating plate 4 to 5 mm beyond the edges of the sheets.
Therefore, even in an ordinary cassette and a universal cassette, when the number of loaded sheets decreases, the sheets tend to get shifted. In particular, when the sheet is smaller than the standardized size, or when a sheet feed roller having a small diameter is used, one end of the sheets and the sheet feed roller tend to get separated by a large distance, so that the sheets tend to be improperly fed.